This invention pertains generally to switching arrangements, and particularly to switching arrangements using transistors as switching elements.
It is well known in the art that so-called electronic switches (or gates) are useful in many applications wherein high frequency signals are to be switched (or gated). For convenience, reference will hereinafter be made only to switches, it being deemed obvious that electronic gates are simply a species of an electronic switch. In such switches, solid state devices, as diodes or transistors, are caused to change from an "ON" condition to an "OFF" condition to pass or to inhibit high frequency signals. To be of any great use in many applications for high frequency signals an electronic switch must exhibit: (1) a high degree of isolation (meaning that the impedance of the solid state devices must be high in the "OFF" condition and low in the "ON" condition); (2) low switching transients (meaning minimal leakage of gating signals into the radio frequency signal path); (3) an appropriate bandwidth (meaning that the frequency of any signal applied to the electronic switch may be changed within reasonable limits); (4) inherent balance (meaning that parameters of the solid state devices need not be closely matched); and (5) independence from complex control circuitry (meaning that the physical size of the electronic switch is kept as small as practicable).
Unfortunately, known electronic switches such as the so-called "balanced pi" configuration cannot simultaneously meet all of the requirements just listed. In particular, if isolation is increased, bandwidth is decreased. High terminating impedances produce adequate isolation with narrow bandwidth; conversely, low terminating impedances produce poor isolation with relatively wide bandwidth.
In many applications, spurious signals generated when the state of a balanced pi switching arrangement is changed, i.e. switching transients, detract substantially from the value of such an arrangement. Thus, even though an intrinsically high degree of symmetry of elements exists in the balanced pi switching arrangement to permit common mode rejection of some switching transients, the bandwidth of any practical balanced pi switching arrangement with low switching transients must be extremely narrow if high isolation and relatively wide bandwidth are both required. An arrangement different from the balanced pi switching arrangement is usually used. For example, diode gates are known. However, such gates require diodes with parameters that are closely matched. Such a requirement means that diodes must be screened before assembly and used in sets.